1. Field
The disclosed concept pertains generally to circuit interrupters and, more particularly, to circuit breakers including an electronic trip mechanism. The disclosed concept also pertains to methods of providing electronic overload protection.
2. Background Information
Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 5,260,676; and 5,293,522.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal which is heated and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature, which is attracted by the sizable magnetic forces generated by a short circuit or fault, also unlatches, or trips, the operating mechanism.
Miniature circuit breakers use bimetals or analog circuits to provide overload (thermal) protection. Known mechanical trip systems physically move bimetals in order that tripping occurs at a fixed bimetal temperature. Bimetals do a good job of simulating thermal cooling of power conductors. The bimetal trips a circuit breaker when its temperature reaches a certain predetermined value. Most of today's circuit breakers are not ambient temperature compensated.
UL 489 is a molded case circuit breaker standard that controls tripping characteristics. For a circuit breaker rated at, for example, 30 A or less, the following performance in Table I (defining a set of thermal overload conditions for a circuit breaker (molded case circuit breaker standard) section 7.1.2 “Calibration Tests”) is required at different current levels relative to the rated current:
TABLE 1IshuntTime (t) at Ishunt valueTrip?=250%10 seconds < t < 180 secondsyes=250%t < 10 secondsno=200%12 seconds < t < 120 secondsyes=200%t < 12 secondsno=135%t < 60 minutesyes<=110% must not tripno
The two 250% thermal overload conditions of Table 1 are for a post short circuit event of a circuit breaker.
Analog circuits can simulate cooling using charge stored on a capacitor, which is simply reset to a fixed thermal level after a trip. See, for example, U.S. Pat. No. 5,418,677.
Some analog circuits may use the temperature of an internal shunt for tripping, but this technique suffers from ambient temperature calibration issues or inaccuracies at the, above, 135% must trip setting of UL 489.
U.S. Pat. No. 7,675,721 discloses a processor including a thermal overload predictive function, a shunt wire structured to measure current flowing through separable contacts for the thermal overload predictive function, and a temperature sensor structured to measure the temperature of the shunt wire. The thermal overload predictive function receives the measured current and the measured temperature of the shunt wire, and the processor, responsive to the thermal overload predictive function, causes an operating mechanism to trip open the separable contacts in response to the measured current and the measured temperature of the shunt wire. The shunt wire has a wire gauge about the same as the wire gauge of a corresponding power circuit wire. The processor responds to the thermal overload predictive function to cause the operating mechanism to trip open the separable contacts when the thermal overload predictive function determines that the measured current is greater than a first predetermined value, and the measured temperature of the shunt wire is greater than a second predetermined value.
The physical properties of thermal mass and thermal resistance determine trip-time characteristics.
Prior known electronic-mechanical systems sense the temperature of a fixed impedance conductor and trip at a fixed temperature with the time delay determined by the conductor's thermal mass.
There is room for improvement in circuit interrupters, such as circuit breakers, providing electronic overload protection.
There is also room for improvement in methods of providing electronic overload protection.